Digital rights group Public Knowledge plans to file mobile traffic-throttling complaints against big four U.S. carriers

Digital rights group Public Knowledge will file Net neutrality complaints against each of the four largest mobile carriers in the United States over their practice of throttling some traffic, in some cases on so-called unlimited data plans.

Public Knowledge on Wednesday sent letters to AT&T, Verizon Wireless, Sprint, and T-Mobile USA, telling the carriers it plans to file traffic-throttling complaints at the U.S. Federal Communications Commission. The letters are the first step toward filing a formal complaint with the FCC.

The complaints will focus on practices at AT&T, Verizon, and Sprint of throttling mobile data subscribers who pay for unlimited data plans and T-Mobile's practice of exempting network-speed-test app Ookla from throttling after subscribers reach their data cap, thus disguising their throttled speeds.

The carriers' practices violate some parts of the FCC Net neutrality rules that survived a court challenge earlier this year, Public Knowledge said in a press release.

Sprint and Verizon violate the FCC's Net neutrality transparency rule by failing to "meaningfully disclose which subscribers will be eligible for throttling," the group said.

AT&T, Sprint, and Verizon violate the transparency rule by failing to disclose which areas of the network are congested and are subject to throttling, Public Knowledge said. T-Mobile violates the transparency rule by preventing throttled subscribers from determining the actual network speed available to them, it said.

"If the FCC's transparency rules mean anything, they must require carriers to let subscribers know why, when, and to what speed their connections might be throttled," Public Knowledge Vice President Michael Weinberg said in a statement.

"Today, Sprint and Verizon subscribers will not know if they are eligible for throttling until after they have crossed the usage threshold," he added. "AT&T, Sprint, and Verizon subscribers will not know they will be throttled until they are actually connected to a congested cell site. T-Mobile subscribers do not know the actual speed of their throttled connection. This is far from transparent."

Sprint and Verizon must publish monthly data thresholds showing when subscribers face throttling in order to comply with the FCC's transparency rule, and AT&T, Sprint, and Verizon must publish real-time information about parts of their network that are congested enough to trigger throttling, Public Knowledge said.

The information must be available in open and accessible formats, the group said.

The practice of mobile data throttling came to the forefront after Verizon announced in July that it planned to slow the traffic of the top 5 percent of LTE data users.